In the case of such a known device, a dry process for separating the wet paint overspray from the respective stream of crude gas is effected after a fluidic particle-like filter aid material referred to as a “precoat” material has been added to the stream of crude gas.
This filter aid material serves as a barrier layer that is deposited on the surfaces of the filter elements in order to prevent these surfaces from sticking together due to overspray particles adhering thereto. By periodically cleaning the filter elements, the mixture consisting of filter aid material and wet paint overspray from the filter elements ends up in filter aid material reservoirs from which it can be sucked out in order to be disposed off or reused as a filter aid material. Furthermore, the mixture of filter aid material and wet paint overspray located in the filter aid material reservoir can be fluidised by means of pulses of compressed air issuing from a compressed air lance so as to cause it to rise up out of the filter aid material reservoir towards the filter elements where it is then deposited.
A critical point for the reliability of the process when using such a device for separating wet paint overspray is that of the flow properties of the mixture consisting of filter aid material and wet paint overspray. If the flow properties are no longer adequate, then an exchange of material can no longer take place in the filter aid material reservoirs. The material in the filter aid material reservoirs can no longer flow to the extraction opening and the extent to which the containers are filled will remain above the value which terminates the suction process. In this case, the painting process has to be interrupted and the material in the filter aid material reservoirs must be loosened manually in such a way as to restore the fluidity thereof and thereby enable it to be sucked out.
Even if the filter aid material reservoirs are provided with fluid bases in the form of plates consisting of a sintered synthetic material so as to enable the material contained in the filter aid material reservoirs to be fluidised by supplying compressed air thereto, then sufficient loosening of the material for the purposes of re-establishing the desired flow properties of the material cannot be ensured thereby. Namely, the adherent properties of the particles in the mixture consisting of filter aid material and wet paint overspray have a substantially greater effect than the forces produced by the flow of the compressed air so that the layer formed by the material is lifted as a whole or else channels are formed in the material through which the compressed air flows upwardly. In addition, the process of fluidising the material is made more difficult due to the large spread in the size distribution of the particles in the filter aid material (in a range of approximately 2 μm to approximately 100 μm). In order to fluidise a fill of particles having a diameter of 2 μm to a porosity of approximately 0.85, it suffices to have a flow rate of 0.00016 m/s. For the purposes of fluidising a fill of particles having a diameter of 100 μm, one needs a flow rate of 0.34 m/s, i.e. an approximately 2,000 times higher rate of flow than is the case for particles having a diameter of 2 μm. Consequently, even if the forces produced by the flow should outweigh the adhesive forces, uniform fluidisation conditions cannot be established merely by feeding in compressed air. Rathermore, a sort of classifying process will take place, whereby the fine components will be carried away but the coarse components will remain immovable on the bottom of the filter aid material reservoir. This classifying effect also occurs if the filter aid material is whirled up by bursts of compressed air by means of fluidising devices arranged above the filter aid material reservoirs.